2159
The Journal of Experimental Biology 203, 2159–2170 (2000)
Printed in Great Britain © The Company of Biologists Limited 2000
JEB2664
TERRESTRIAL LOCOMOTION IN THE BLACK-BILLED MAGPIE: KINEMATIC
ANALYSIS OF WALKING, RUNNING AND OUT-OF-PHASE HOPPING
MYRIAM VERSTAPPEN*, PETER AERTS AND RAOUL VAN DAMME
Department of Biology, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Antwerp, Belgium
*e-mail: vstappen@uia.ua.ac.be
Accepted 27 April; published on WWW 22 June 2000
Summary
The inter-limb kinematic patterns of walking, running
the legs are believed to have different functions. The
and out-of-phase hopping in black-billed magpies (Pica
hindlimb kinematic patterns of magpies are like those of
pica) were studied using high-speed video recordings. The
other flying and more terrestrial bird species; however,
flexion/extension patterns of the joints were similar
striking differences are found in comparison with humans
between the gait types, suggesting that the within-leg
at the level of the internal angles. This is probably due to
control of the angular excursions is similar. This result is
the differences in the morphology and configuration of
further supported by the fact that running and hopping are
their legs.
alternative gaits at speeds higher than walking; however,
magpies show a preference for hopping. Moreover, only
small differences occur between the kinematic patterns of
Key words: kinematics, bipedal gait, terrestrial locomotion, blackthe two limbs during out-of-phase hopping, during which
billed magpie, Pica pica, running, hopping, walking.
Introduction
Terrestrial locomotion in birds can involve several different
gaits. Birds can walk, run or hop (in- or out-of-phase;
Verstappen and Aerts, 2000), but most species use only one or
two gait types. Several studies have already dealt with
kinesiological aspects of these different gait types (energetics,
see, Bamford and Maloiy, 1980; Brackenbury and Avery,
1980; Brackenbury et al., 1981, 1982; Cavagna et al., 1977;
Fedak et al., 1974; Fedak and Seeherman, 1979; Muir et al.,
1996; Pinshow et al., 1977; Roberts et al., 1997, 1998a; Taylor
et al., 1971; Taylor, 1977; mechanics, see Alexander et al.,
1979; Clark, 1988; Clark and Alexander, 1975; Fedak et al.,
1982; Heglund et al., 1982a,b; Roberts et al., 1998b; Taylor et
al., 1982; electromyography, see Jacobson and Hollyday,
1982a,b; Gatesy, 1999b; Weinstein et al., 1983; neurology, see
Jacobson, 1980; Johnston and Bekoff, 1996; Steeves et al.,
1987) but, compared with the wealth of studies dealing with
mammalian locomotion, this number, and thus our knowledge
of terrestrial bird locomotion, is relatively small.
Verstappen and Aerts (2000) analysed the spatio-temporal
gait characteristics of walking, running and out-of-phase
hopping in the black-billed magpie (Pica pica). With
increasing speed, the three gait types do not occur successively
as walking, trotting and galloping do in quadrupedal mammals.
In magpies, walking is used at low velocities, and running and
out-of-phase hopping are alternative gaits for higher speeds,
with birds showing a preference for hopping. This is
comparable with the situation in humans, hence the fact that
humans prefer running at higher speeds (Caldwell and Whitall,
1995; Minetti, 1998). Verstappen and Aerts (2000) concluded
that no intricate shifts in control and coordination are required
for the gait transitions in magpies. They postulate that running
emerges from ‘over-powered’ walking, and that hopping is like
running but with a reduced phase difference between the limbs.
From this, it follows that inter-limb kinematic patterns should
be fairly similar over the different gait types. During out-ofphase hopping, the legs are not set down simultaneously. It is
postulated that a different function might be present for the two
legs (Verstappen and Aerts, 2000). Moreover, it is not obvious
why the birds use running and hopping as alternative gaits and
why they prefer hopping at higher speeds. In the light of the
‘coupled oscillators for the legs’ theory (see Kugler and
Turvey, 1987; Peck and Turvey, 1997), it can be postulated
that intrinsic features of the bird’s build (e.g. body proportions)
normally evoke out-of-phase hopping at speeds greater than
walking, but that subtle changes in the initial state of the
neuromuscular system are sufficient to shift the point at which
walk–run coordination becomes unstable towards higher
speeds (Verstappen and Aerts, 2000). To explain all these
differences, more information on the detailed angular
displacements of the leg and its segments over the course of a
stride, i.e. the kinematics, in birds is needed.
The few studies on birds that concentrate on the detailed
kinematics of the legs during terrestrial locomotion have
mostly used conventional video recordings (50–60 frames s−1)
(e.g. Dagg, 1977; Jacobson and Hollyday, 1982a; Johnston and
Bekoff, 1992; Miller, 1937; Muir et al., 1996; Stolpe, 1932),
2160 M. VERSTAPPEN, P. AERTS AND R. VAN DAMME
which may suffice for walking, but high-speed video
recordings are essential for the faster gaits (running and
hopping). Furthermore, analyses are often limited to one or two
gaits (walking and/or running, e.g. Cracraft, 1971; Dagg, 1977;
Jacobson and Hollyday, 1982a; Manion, 1984; Rylander and
Bolen, 1974) or to a selection of joint angles (e.g. Dagg, 1977:
only the ankle; Gatesy, 1999a: no ankle). To our knowledge,
no study has compared quantitative data (internal and external
angles) for hindlimb movements during walking, running and
out-of-phase hopping in birds. The present study, therefore,
aims to give a full kinematic description of the external and
internal joint angles during walking, running and out-of-phase
hopping in the black-billed magpie.
overall picture of the locomotor cycle is compiled from
several fragments. To give further support to the description
of the angular displacement patterns of the hindlimb joints
throughout a complete stride, the birds were also filmed at a
wider view. For this purpose, a conventional camera
(50 frames s−1, Panasonic F15) was used.
In this study, gaits are distinguished by footfall patterns, so
that they can be identified directly from the video recordings
(see also Verstappen and Aerts, 2000). The onset of the stance
phase coincides with the moment the hallux (=hind-toe, digit
1) touches the ground (see Figs 1B, 7). When the third digit
leaves the ground, the recovery phase begins (see Figs 1B, 7).
Walking is defined as a symmetrical gait with two doublesupport phases in each stride (=from the lift-off of a foot until
the next lift-off of the same foot). During running, floating
phases replace the double-support phases (high-speed filming
is necessary to record these phases). Hopping, which also
includes an aerial phase, is an asymmetrical gait and is thus
easily differentiated from running. Out-of-phase hopping is
also known as bipedal galloping (see Whitall and Caldwell,
1992; Caldwell and Whitall, 1995) or skipping (see Minetti,
1998). On the basis of the continuity of the gait and to obtain
a velocity range, 28 walking, nine running and 32 out-of-phase
hopping high-speed video sequences were selected for further
analysis. From the conventional video recordings (50 Hz),
seven walking sequences that included at least one complete
stride were also analysed. This temporal resolution proved to
be sufficient to obtain relevant additional data for the slowest
gait. The velocity of the birds in each selected sequence was
calculated by determining the slope of the linear regression of
the forward displacement of the eye against time (all r2>0.98).
The circles in Fig. 1A mark the body points that were used
for digitisation: bill tip (b), eye (e), neck (n), hip (h), knee (k),
ankle (a), inter-phalangeal joint (i), and the distal ends of the
hallux (ha) and the front-toes (f). These body points were
always digitised on the side of the body closest to the camera.
Materials and methods
Initially, three hand-raised black-billed magpies (Pica pica
L.) were trained for several months to move on a treadmill to
gather high-speed recordings of complete locomotion cycles
(strides) in close up. These locomotion bouts were very
irregular and highly unrepeatable because the animals tended
to flap their wings, so these results and this technique had to
be discarded. The animals were therefore placed on a 6 m long
outdoor running track where they were encouraged to move at
different speeds to provide kinematic data over a range of
speeds. To prevent slipping, the running track was covered
with a thin layer of cork. To obtain a better view of the
proximal hindlimb joints, the birds’ feathers at the hip and knee
joints were cut away a few days prior to filming. This did not
seem to affect their behaviour in any way.
To obtain accurate information about the positions of the
joints and the placing of the toes, high-speed video recordings
(500 frames s−1, NAC1000) were taken in close up. The
camera settings were such that the image of the bird almost
filled the screen. As a result, only parts of complete strides
could be kept in the field of view, which means that the
A
e
b
B
n
H
T
h
F
K
a
TB
A
ha
TM
i
I
k
f
D
Retraction Protraction
Fig. 1. (A) Outline of a magpie showing the points digitized (circles) and the external and internal angles. Lower case letters represent the body
points: a, ankle; b, bill tip; e, eye; f, front toes; ha, hallux; h, hip; i, interphalangeal joint; k, knee; n, neck. Upper case letters represent the
angles: A, ankle; D, digits; F, femur; H, hip; I, inter-toe; K, knee; T, trunk; TB, tibiotarsus; TM, tarsometatarsus. (B) Retraction and protraction
angles of the leg.
Kinematics of bipedal gaits in magpies 2161
hindlimb during this phase, leading to muscular and skeletal
stresses much higher than those present during the recovery
phase. (iii) Recovery phase duration is found to be independent
of speed and to be similar for the three gaits, which suggests
passive mechanical control (Verstappen and Aerts, 2000). For
these reasons, high-speed sequences of primarily stance phases
were used in further analysis.
After digitisation, the displacements (in m) of the
coordinates against time were obtained. Next, time histories of
angular displacements were calculated. A zero-phase-shift
fourth-order Butterworth filter was used to remove noise from
the angular displacement data. The external angles of the trunk,
femur, tibiotarsus and tarsometatarsus were measured between
these body segments and the horizontal (see Fig. 1A). The
internal angles of the hip, knee and ankle were measured
between two body segments (see Figs 1A, 7). The movements
of the toes are expressed by an external angle measured
between a line connecting digit 3 with the hallux and the
horizontal, and by an internal angle measured between digit 3
and the hallux (inter-toe angle) (see Figs 1A, 7).
The angular displacement patterns can show several maxima
and minima throughout a stride (e.g. see Fig. 2). These
maxima/minima are coded according to the phase in which
Recovery
phase
rmaTM
A
Recovery
phase
Stance
phase
F
External angle
Extension
In the case of the high-speed recordings, the coordinates of
these body points were digitised at 250 frames s−1 using APAS
software (Ariel Performance Analysis System, Ariel Dynamics
Inc.). The sequences obtained using conventional video were
digitised at 50 frames s−1 with custom-designed software. To
reduce the error involved in the digitisation of the coordinates
of the hip and knee (due to the presence of the wing feathers),
an overlay stick model of the bird (based on X-ray photographs
and video) drawn on a transparency was used. Whenever a
joint was not clearly visible, this transparency was
superimposed on the screen to locate it more precisely. Despite
these precautions, the digitization errors for the hip, knee and
ankle are expected to be larger than those for the other body
points. However, none of the errors was so large that the
angular patterns presented here are not a valid representation
of the motions of the legs during locomotion.
In the present analysis, most attention was paid to the
angular changes of the hindlimb segments during the stance
phase for the following reasons. (i) It has previously been
found that most of the metabolic cost during running in bipeds
(birds and humans) is inversely correlated with the time
available to generate force, i.e. the duration of the stance phase
(Roberts et al., 1998b). (ii) Ground reaction forces act on the
TM
rsmaTB
TB
smiTM
rmiTB
B
Recovery rsmaA
phase
Recovery
phase
Stance
phase
smaA
A
rsmaK
Internal angle
K
smiA
smaK
H
smiK
rmiA
rmiK
Flexion
Fig. 2. Schematic representation of changes in
external angles (A) and internal angles (B) with
time for all gaits derived from real data (50 Hz).
The stick figures, representing the leg closest
to the camera, refer to the following angles/
moments from left to right: rmiK, rmiA, onset
of stance phase, smiA, smaA and onset of
recovery phase. The abbreviations for the angles
are defined in Fig. 1A. The maxima/minima in
the measures are coded according to the phase
in which they occur: r, recovery; s, stance; rs,
recovery–stance transition; sr, stance–recovery
transition; and also whether they represent
maxima (ma) or minima (mi). See Materials and
methods for further details.
Extension
Flexion
T
-0.3
-0.2
-0.1
0
0.1
0.2
Time (s)
0.3
0.4
0.5
0.6
2162 M. VERSTAPPEN, P. AERTS AND R. VAN DAMME
160
200
B
150
Internal angle (degrees)
External angle (degrees)
A
F
100
TM
T
50
TB
0
-0.05
0
0.05
0.1
Time (s)
0.15
0.2
A
120
100
H
80
K
60
Onset of
recovery
Onset of
stance
-50
-0.1
140
Onset of
recovery
Onset of
stance
40
0.25
-0.1
-0.05
0
0.05
0.1
Time (s)
0.15
0.2
0.25
Fig. 3. External (A) and internal (B) angles recorded during the stance phase and part of the recovery phase during walking. Abbreviations are
as in Fig. 1A.
they occur. The first letter of the code indicates the phase in
which the maximum or minimum occurs: r for recovery, s for
stance. The next two letters express whether it concerns a
maximum (ma) or a minimum (mi). Upper case letters indicate
the joint or leg segment (H, hip; K, knee; A, ankle; I, inter-toe;
TB, tibiotarsus; TM, tarsometatarsus; D, digits; F, femur; T,
Trunk). For example, rmiA is the minimum internal angle
reached by the ankle during the recovery phase; smaTB is the
maximal external angle reached by the tibiotarsus during the
stance phase. If a minimum or maximum occurs at the
transition from recovery to stance (or vice versa), it is indicated
by ‘rs’ (recovery–stance transition) or ‘sr’ (stance–recovery
transition) (e.g. rsmaA is the maximum angle reached by the
ankle at the recovery–stance transition).
To determine whether the relative timing of the
maxima/minima in a stride (divided into stance and recovery
phases) changes with increasing velocity, both phase durations
were set to 100 %, with the start and end of each phase set
at 0 and 100 %, respectively. The correlation between the
maxima/minima and relative timing and the velocity was
200
Onset of
stance
150
100
50
160
Onset of
recovery
B
F
TM
T
TB
0
-0.15
Results
General description
Flexion and extension patterns of the internal angles are
accomplished through complex interactions of rotations of the
different leg segments (external angles). Comparison of all the
digitised sequences showed that the flexion–extension cycles
were basically the same for walking, running and out-of-phase
Internal angle (degrees)
External angle (degrees)
A
described using least-squares linear regression (Statistica 5.0).
Analysis of covariance (ANCOVA) was used to compare
maxima/minima and relative timings between the three gaits
(Statistica 5.0).
During the asymmetrical hopping gait, the two legs
apparently perform different functions: braking or propulsion.
The angular displacements were therefore determined for the
landing (first leg to make ground contact, first to leave the
ground) and take-off (second leg to make ground contact,
second to leave the ground) leg separately and then compared
using t-tests (Statistica 5.0).
140
120
100
80
H
A
60
-0.1
-0.05
0
0.05
Time (s)
0.1
0.15
0.2
40
-0.15
Onset of
recovery
Onset of
stance
K
-0.1
-0.05
0
0.05
Time (s)
0.1
0.15
0.2
Fig. 4. External (A) and internal (B) angles recorded during the stance phase and part of the recovery phase during running. Abbreviations are
as in Fig. 1A.
Kinematics of bipedal gaits in magpies 2163
Extenal angle (degrees)
140
160
Onset of
recovery
A
B
F
120
100
Internal angle (degrees)
160
TM
80
60
T
40
Onset of
stance
20
0
-0.1
-0.05
0
TB
0.05
Time (s)
0.1
0.15
140
120
100
H
A
80
60
40
-0.1
0.2
Onset of
stance
-0.05
0
Onset of
recovery
0.05
Time (s)
K
0.1
0.15
0.2
Fig. 5. External (A) and internal (B) angles recorded during the stance phase and part of the recovery phase during out-of-phase hopping:
landing leg. Abbreviations are as in Fig. 1A.
hopping. However, the maximum/minimum angular values and
their relative timing within the stance or recovery phases
differed between the gaits (see below). Fig. 2 presents schematic
representations of the observed angular displacements derived
from sequences digitised at 50 Hz. On the basis of Figs 2–6, a
general description of these patterns of external and internal
angles during the stance and recovery phases is given below.
Stance phase
At the onset of the stance phase, the knee and ankle start
to flex. The knee generally continues to flex throughout the
rest of this phase (a slight extension occurs just before the
end of the stance phase because of a small temporary increase
in the tibiotarsus angle, after which flexion is resumed). Near
mid-stance, the ankle angle reaches a minimum (because of
the rapid decrease in the tibiotarsus angle), after which it
extends again. This extension is caused by the rapid decrease
in the angle of the tarsometatarsus. At the late stance phase,
just prior to the onset of recovery, the ankle starts to flex
again as a result of a decrease in the tibiotarsus angle and an
Recovery phase
The flexion of the ankle, knee and hip that started during the
stance phase continues for the beginning of the recovery phase.
The flexion of the knee reverses to rapid extension after
approximately one-third of the recovery phase. At
approximately mid-recovery, the ankle reaches a minimum
value and starts to extend. The extension of the hip starts near
the end of this phase, before the foot touches the ground.
Because of the small changes in the trunk and femur angles, the
angular displacements of the hip are small. The initial flexion of
the knee is accomplished through a decrease in the tibiotarsus
angle, and the initial flexion of the ankle is accomplished
through a decrease in the tibiotarsus angle and an increase in the
angle of the tarsometatarsus. The knee extends as a result of a
180
A
150
F
100
TM
T
50
TB
0
-50
-0.05
B
160
Internal angle (degrees)
External angle (degrees)
200
increase in the tarsometatarsus angle. Although the angular
displacements at the hip are small, the hip generally extends
throughout the entire stance phase, extension reaching a
maximum at approximately the transition from stance to
recovery.
Onset of
stance
0
0.15
120
0.2
H
100
80
60
Onset of
recovery
0.05
0.1
Time (s)
140
40
-0.05
A
Onset of
recovery
Onset of
stance
0
0.05
0.1
Time (s)
0.15
K
0.2
Fig. 6. External (A) and internal (B) angles recorded during the stance phase and part of the recovery phase during out-of-phase hopping: takeoff leg. Abbreviations are as in Fig. 1A.
2164 M. VERSTAPPEN, P. AERTS AND R. VAN DAMME
Table 1. Maxima/minima (in degrees) and relative timings (in %) of the angles measured for walking, running and hopping
magpies
Walking
Mean
Running
Hopping
(landing leg)
Hopping
(take-off leg)
Differences among
means of intercepts
N
Mean
N
Mean
N
Mean
N
F
Maxima/minima (degrees)
rmiK
50±12
rsmak
143±9
smiK
72±12
smaK
85±17
rmiA
62±10
rsmaA
151±7
smiA
108±10
smaA
142±10
rmiTB
4±5
rsmaTB
116±6
rmaTM
173±7
smiTM
*
20
34
26
26
22
35
34
28
21
35
30
27
*
148±13
79±13
89±15
64±13
145±13
108±11
144±25
2±6
115±8
*
63±16
5
9
6
5
8
9
7
6
4
9
9
6
*
146±6
70±13
85±20
76±9
*
103±8
138±16
0±4
*
155±11
*
6
9
10
8
7
11
12
12
8
9
6
12
45±13
143±20
92±14
103±15
70±9
*
102±8
152±11
14±11
120±10
172±26
66±10
14
14
22
21
10
15
17
19
14
14
10
17
2.58
0.47
10.72
5.09
4.01
1.17
1.99
3.33
8.07
3.12
3.58
3.18
Relative timing (%)
rmiK
27.3±7.9
rsmaK
3.5±2.6
smiK
*
smaK
84.6±8.3
rmiA
52.3±9.3
rsmaA
4.8±2.4
smiA
44.1±8.3
smaA
86.8±6.0
rmiTB
16.2±9.6
rsmaTB
1.6±2.9
rmaTM
72.7±5.1
smiTM
90.0±2.3
16
26
24
26
16
29
27
27
16
27
16
27
‡
‡
55.1±2.4
83.4±6.3
‡
2.8±1.9
41.4±4.4
84.0±6.0
‡
0.3±0.8
‡
93.7±5.0
0
0
5
4
0
8
7
5
0
6
0
5
28.9
1.5±3.4
57.7±4.6
84.6±5.5
44.4
2.4±3.4
42.8±3.4
*
22.2
0.4±2.22
66.7
96.5±3.1
1
5
9
7
1
9
11
11
1
7
1
10
34.3
0.2±1.5
*
82.0±7.1
48.6
1.5±2.5
*
89.4±3.0
22.9
0±1.4
71.4
*
1
9
17
15
1
14
17
14
1
9
1
14
P
0.07
0.71
0.00001
0.003
0.01
0.33
0.12
0.03
0.0002
0.03
0.02
0.03
0.52
0.43
0.40
0.7
0.7
0.7
6.14
0.29
2.03
0.001
0.8
0.1
1.26
0.3
4.17
0.01
Differences
among slopes
F
P
2.58
0.07
1.10
0.4
2.65
2.67
5.91
0.06
0.06
0.001
0.36
3.77
0.8
0.02
1.76
2.86
0.27
0.05
5.60
0.002
Values are means ± S.D., N is number of observations.
For an explanation of the abbreviations, see Figs 1 and 2 and Materials and methods.
F and P values in the last two columns are derived from analyses of covariance testing for differences among gaits. For variables that were
affected by speed in one of the gaits, we compared the slopes and intercepts of the regression lines among gaits. For variables that did not
change with speed, differences among means were tested by one-way ANOVA.
*Means were not calculated because the variable depends on speed (see Table 2).
‡No data available.
rapid increase in the tibiotarsus angle, while the extension of the
ankle is achieved by an increase in the tibiotarsus angle and a
decrease in the tarsometatarsus angle. At the switch between the
recovery and stance phases, the ankle and knee show maximal
extension. As a result, the tibiotarsus angle is largest at this
transition.
Effects of speed
Walking velocities between 0.5 and 1.2 m s−1 were
measured. The animals ran at velocities between 1.3 and
2.4 m s−1 and hopped at velocities ranging from 1.1 to 2.3 m s−1.
Maxima/minima
For most of the angles measured, the maxima/minima were
unaffected by speed (regression analysis, P>0.05; Table 1).
Exceptions to this rule (Table 2) are discussed below.
The minimum angle reached by the tarsometatarsal segment
at the end of the stance phase (smiTM) during walking
decreased significantly with speed (r2=0.3, P=0.003). From the
regression equation (Table 2), increasing the walking velocity
from 0.5 to 1.2 m s−1 would result in a decrease in smiTM from
71 to 57 °.
During the recovery phase of running, the minimum angle
reached by the knee (rmiK, r2=0.85, P=0.03) and the maximum
angle reached by the tarsometatarsus (rmaTM, r2=0.68,
P=0.006) varied with speed. The respective regression
equations (Table 2) predict that a change in running velocity
from 1.3 to 2.4 m s−1 would result in an increase in rmiK from
36 to 78 ° and an increase in rmaTM from 170 to 188 °.
Four angles of the landing leg change with velocity during
hopping. As in running, the minimum knee angle during the
recovery phase (rmiK) increases with velocity (r2=0.79,
P=0.02). Here, a speed increment from 1.1 to 2.3 m s−1 is
predicted to cause a change in rmiK from 30 to 84 °. At the
Kinematics of bipedal gaits in magpies 2165
Table 2. Intercepts and slopes of the linear least-squares
regression lines relating maxima/minima and relative timings
of angles to speed of locomotion
Maxima/minima
Walking
smiTM
Running
rmiK
rmaTM
Hopping
(landing leg)
rmiK
rsmaA
rsmaTB
smiTM
Hopping
(take-off leg)
rsmaA
Relative timing
Walking
smiK
Hopping
(landing leg)
smaA
Hopping
(take-off leg)
smiK
smiA
smiTM
S.E.M.
r2
P
−21.9
6.6
0.3
0.003
13.8
7.5
38.1
16.6
9.4
4.3
0.85
0.68
0.03
0.006
−19.5
178.1
125.5
108.7
16.4
11.4
4.3
10.6
44.9
−18.9
−8.9
−31.6
11.4
7.1
2.7
6.5
0.79
0.44
0.6
0.7
0.02
0.03
0.01
0.0007
168.5
10.1
−14.4
5.9
0.31
0.02
79.9
4.5
−21.3
5.4
0.41
0.0007
72.4
4.4
9.9
2.7
0.61
0.005
Intercept
S.E.M.
81.9
5.3
−13.1
148.6
80.3
52.8
86.6
8.4
4.0
3.4
Slope
−14.8
−5.6
5.7
4.8
2.3
2.0
0.39
0.28
0.40
0.008
0.03
0.01
Also shown are the standard errors (S.E.M.) for both parameters,
the coefficient of determination (r2), and its significance (P).
Only those variables that were significantly affected by speed are
shown.
For an explanation of the abbreviations, see Figs 1 and 2 and
Materials and methods.
transition from recovery to stance phase, the maximum
angles of the ankle (rsmaA, r2=0.44, P=0.03) and the
tibiotarsus (rsmaTB, r2=0.60, P=0.01) decrease slightly with
speed. An increase in velocity from 1.1 to 2.3 m s−1 changes
rsmaA from 157 to 135 ° and rsmaTB from 116 to 105 °. The
minimum angle reached by the tarsometatarsus during the
stance phase (smiTM) decreases dramatically with increasing
speed (r2=0.70, P=0.0007). With a velocity increase from 1.1
to 2.3 m s−1, smiTM decreases from 74 to 36 °. In the takeoff leg, only rsmaA changes significantly with velocity
(r2=0.31, P=0.02). As for the landing leg, rsmaA decreases
slightly with increasing locomotor speed in the take-off leg.
It changes from 153 to 135 ° as the speed increases from 1.1
to 2.3 m s−1.
Relative timings
For most angles, the relative timing of the maxima and
minima did not vary significantly with velocity (Table 1). Only
the exceptions to this rule (Table 2) are discussed below. Note
that the observed relative timings for running are not related
significantly to speed (Table 1); this may be partly due to the
low number of observations for this gait type.
For walking, the only exception is the relative timing of the
minimum knee angle during the stance phase (smiK, r2=0.41,
P=0.0007). As the velocity increases, this angle is reached
earlier in the phase. At a velocity of 0.5 m s−1, it is reached after
70 % of the stance phase, while at a velocity of 1.2 m s−1, the
minimum occurs at 54 % of the phase.
For the landing leg during hopping, the maximal ankle angle
is attained at a later stage in the stance phase at higher speeds
(smaA, r2=0.61, P=0.005). The ankle reaches its maximal
angle after 83 % of the total duration of the stance phase at
1.1 m s−1 and after 95 % at 2.3 m s−1. In the take-off leg, speed
modifies the relative timing of the minimal angle of the knee
(smiK, r2=0.39, P=0.008), of the ankle (smiA, r2=0.28,
P=0.03) and of the tarsometatarsus (smiTM, r2=0.40, P=0.01)
during the stance phase. At a hopping velocity of 1.1 m s−1, the
minimum knee angle is reached after 64 % of the total duration
of the stance phase, while at 2.3 m s−1, it occurs after 46 %. For
a similar increment in velocity, the minimum ankle angle
changes its relative timing from 47 to 40 %, and the minimal
tarsometatarsus angle changes its relative timing from 93 to
100 %.
Differences between the kinematics of the landing and take-off
leg during hopping
During hopping, the maxima/minima of the following four
angles differ between the landing and the take-off leg (Table 1,
t-tests): smaA, t29=2.94, P=0.006; smiK, t30=4.08, P=0.003;
smaK, t27=2.59, P=0.02; and rmiTB, t20=3.33, P=0.003. These
minima and maxima have larger values in the take-off leg than
in the landing leg. The average difference is approximately 14 °
for smaA and rmiTB, 18 ° for smaK and 22 ° for smiK. In
addition, rsmaTB also takes higher values in the take-off leg
than in the landing leg, regardless of the effect of speed
(ANCOVA with speed as covariate, difference between legs:
F1,20=10.29, P=0.004). For the other angles, the difference
between the landing and take-off leg was not significant (t-tests
or, if angles change with speed, ANCOVAs: all P>0.08). None
of the relative timings differed significantly between the two
legs (t-tests or ANCOVAs, all P>0.1).
Differences between gait types
Maxima/minima
Analysis of covariance suggested substantial variation
among gaits in smiK, smaK and rmiTB and smaller differences
in rsmaTB, smiTM and smaA (Table 1). However, post-hoc
comparisons show that most of the variation in these angles is
due to the aberrant behaviour of the take-off leg during
hopping. The values for smiK, smaK, smaA and rmiTB are all
similar for walking, running and the landing leg during
hopping, but are higher for the take-off leg. Values for rmaTM
attained during walking do not differ from those for the takeoff leg during hopping, and both are similar to the range of
2166 M. VERSTAPPEN, P. AERTS AND R. VAN DAMME
Table 3. Mean values of the hip, femur and trunk (pelvic pitch) angles during the stance phase
Hopping
Walking
Hip angle (degrees)
Femur angle (degrees)
Trunk angle (degrees)
Running
Landing leg
Take-off leg
Mean
N
Mean
N
Mean
N
Mean
N
82.1±8.0
142.1±9.5
44.3±7.2
27
27
27
82.1±7.4
141.7±13.9
46.0±10.3
5
5
5
93.5±17.8
135.0±15.4
10
8
49.2±6.6
94.9±12.7
137.5±14.3
16
18
21
Values are means ± S.D., N is number of observations.
rmaTM observed in birds running at speeds between 1.3 and
2.4 m s−1. However, lower values of rmaTM are noted for
the landing legs in hopping magpies. Differences in the
speed-dependence of smiTM among gaits prevent direct
comparisons. The minimal tarsometatarsus angle during the
stance phase (smiTM) does not change with speed during
running and in the take-off leg, but decrease slightly during
walking and in the landing leg. Finally, rmiA does not differ
between walking and running and in the take-off leg, but
attains slightly higher values in the landing leg of hopping
magpies.
Relative timings
The relative timings of rsmaA are similar for walking and
running birds and for the landing leg during hopping.
However, the maximum is reached at an earlier stage by the
take-off leg during hopping. The timing of smiK during
running and for the landing leg is similar and, as indicated
above, independent of speed in these cases. During walking,
and for the take-off leg, smiK occurs earlier in the stance phase
at higher speeds. The effect of speed is somewhat greater
during walking. The minimal tarsometatarsus angle (smiTM)
occurs relatively early in the stance phase in walking and
running birds, and significantly later in the landing leg during
hopping. For the take-off leg, the timings for smiTM span the
range of timings in the other gaits, depending on speed. At low
speeds (approximately 1 m s−1), smiTM timings for the takeoff leg are similar to those of walking and running magpies,
while at high speeds (approximately 2 m s−1), the timings are
closer to that of the landing leg.
Hip, femur and trunk
The mean trunk angle during the stance phase of walking is
slightly smaller than that during running (44.3 versus 46.0 °),
but differs significantly from that during hopping (49.2 °; t-test,
t46=−2.39, P=0.02). The mean femur angle during the stance
phase does not differ significantly between walking, running
and both legs during hopping (Table 3). The mean hip angle,
in contrast, is similar during walking and running (82.1 °), but
differs between these two gaits and in the take-off leg during
hopping (walk–hop, t-test, t41=−3.02, P=0.006; run–hop, t-test,
t19=−2.23, P=0.047).
Movement of the toes
When a magpie sets its foot down, it does so by placing the
hallux on the ground first (external angle approximately 200 °,
internal angle approximately 145 °, Fig. 7). Next, the front toes
are set down, and the plantar surface of the toes then makes
contact with the ground. The external angle decreases to
approximately 180 ° and the internal angle increases to
approximately 160 °. As the bird lifts its foot for the recovery
phase, it is unrolled, with the hallux leaving the ground first
and the third digit last. At the last moment before the recovery
phase begins, the external angle measures approximately 110 °,
and the internal angle approximately 130 °. During the
recovery phase, the external and internal angles reach values
of approximately 90 ° and approximately 80 °, respectively.
This movement pattern is similar for walking, running and outof-phase hopping.
Movement of the whole leg (protraction and retraction)
The protraction angle (Fig. 1B) remains constant as the
speed increases during walking, running and hopping
(walking, 49±5.2 °, N=25; running, 43±4.0 °, N=11; landing
leg, 43±5.1 °, N=11; take-off leg, 53±4.6 °, N=14) (means ±
S.D.). In contrast, the retraction angle (Fig. 1B) increases with
Stance phase
External
angle
Recovery phase
Fig. 7. Movements of the toes during the stance and
recovery phases.
Internal
angle
Kinematics of bipedal gaits in magpies 2167
increasing velocity (linear regression, Quattro Pro). Over the
velocity range of walking (0.5–1.2 m s−1), the retraction angle
of the leg increased from 27 to 41 ° (r2=0.32, Pslope=0.0033).
As the running speed increases from 1.3 to 2.4 m s−1, this angle
increases slightly from 30 to 39 ° (r2=0.80, Pslope=0.03). For
the landing leg during hopping, an increase in velocity from
1.1 to 2.3 m s−1 doubles the retraction angle from 28 to 56 °
(r2=0.73, Pslope=0.00057). The retraction angle for the take-off
leg tended to increase (from 20 to 32 °) with increasing
velocity, although not significantly so (r2=0.22, Pslope=0.05).
Discussion
Verstappen and Aerts (2000) proposed that the three gait
types used by black-billed magpies are the result of the same
control mechanisms and thus are highly comparable. Running
is therefore an over-powered style of walking, and hopping is
equivalent to running but with an altered phase shift between
the legs. This hypothesis leads to the prediction that the withinleg control of the angular excursions will be identical and thus
that the kinematic patterns of the three gaits should be fairly
similar. The latter prediction seems to be confirmed by the
present results.
A stride consists of two phases: a stance (ground contact)
phase and a recovery (swing) phase. At the onset of the stance
phase, the leg that touches the ground is almost completely
extended and is placed on the ground in front of the body’s
centre of mass. The body is thus decelarated (braking).
Assuming an analogy between magpies and the quail Coturnix
japonica (Clark and Alexander, 1975), the leg is actively flexed
when the ground reaction force passes anterior to the knee
early in the stance phase. However, muscles act to prevent or
counteract this flexion (for electromyography of terrestrial bird
locomotion, see Jacobson, 1980; Jacobson and Hollyday,
1982a,b; Gatesy, 1999b). In the magpie, a slight hip flexion
sometimes occurs early during the stance phase (probably
resulting from the external forces; see above), but the hip
generally starts to extend at the end of the recovery phase and
continues to do so during the entire stance phase. The knee and
ankle both flex. In the chicken Gallus domesticus (see
Jacobson, 1980; Jacobson and Hollyday, 1982a) and in the
guineafowl Numida meleagris (Gatesy, 1999b), it was found
that these movements are counteracted by muscle activity.
During stance, the magpie’s leg displaces backwards relative
to the body. As soon as the body’s centre of mass passes over
the foot, the body is accelerated (propulsion). Throughout the
ongoing knee flexion, the leg continues to move relatively
backwards, while the hip and particularly the ankle stretch to
cause acceleration. Knee flexion lasts until the next recovery
phase and initiates the lift-off of the foot. Ankle and hip flexion
at the end of the stance phase also aid in lifting the foot. Near
the end of the stance phase, the ongoing knee flexion is briefly
interrupted by a small extension (smaK, Fig. 2B), after which
knee flexion is resumed again. This brief extension–flexion
pattern is also observed in pigeons Columba livia (Cracraft,
1971), chickens (Jacobson and Hollyday, 1982a; Johnston and
Bekoff, 1992; Manion, 1984; Muir et al., 1996) and guineafowl
(at high velocities, Gatesy, 1999b). According to the latter
author, activity of the musculus femorotibialis causes this
extension, and its magnitude is correlated with the degree of
hip extension. In the magpie, this knee extension is positively
related to hip extension during walking and for the take-off leg
during hopping.
During the recovery phase, the switch from flexion to
extension in the three joints does not happen simultaneously.
First the knee, then the ankle and finally, just before the next
stance phase, the hip extend. Because of the configuration of
the bird’s leg, the hip must remain flexed so that the leg does
not touch the ground. The forward movement of the parts of
the leg below the knee is mainly the result of the rapid
extension of the knee. If the ankle were to extend
simultaneously with the knee, the toes would touch the ground
during leg protraction. Therefore, ankle extension starts later
in the phase, mainly to allow extension of the leg as far as
possible before the next stance phase. In humans, maximum
hip flexion angle is also reached at the end of recovery, but in
this case, the protraction of the leg is achieved primarily by
angular displacements in the hip (Borghese et al., 1996;
Sutherland et al., 1994).
Some of the maximum and minimum angular values reached
by the joints and segments are related to speed. During
walking, the minimal tarsometatarsus angle at the end of the
stance phase decreases with increasing velocity. As a result,
step length is expected to increase as the bird walks faster, as
described by Verstappen and Aerts (2000).
During running, the minimal knee angle at the beginning and
the maximal tarsometatarsus angle at the end of the recovery
phase correlate positively with speed. In the landing leg during
hopping, the minimal knee angle during the recovery phase
also correlates positively with velocity. At the transition from
recovery to stance phase, the maximal ankle and tibiotarsus
angle both decrease with speed. These latter angular changes
probably decrease the stiffness of the leg as it becomes more
flexed, which reduces the impact at touch-down. In the takeoff leg, only the maximal ankle angle at the transition from
recovery to stance phase decreases with speed. As in the
landing leg, this causes a decrease in leg stiffness at the
beginning of the stance phase. In walking/running guineafowl,
there is a change in the maximum knee angle before touchdown with velocity (Gatesy, 1999a). In magpies, this angle
does not change with velocity in either of the three gait types.
It is remarkable that, during the recovery phases in running
and for the landing leg during hopping, the knee flexes less
when the locomotor speed increases. This implies that the foot
is lifted less high above the ground during the forward swing
of the leg. Consequently, it is conceivable that, at higher
speeds, the toes would touch the ground, which seems
unfavourable from the point of view of stability. During
hopping, however, the body is launched into the air, so that the
potential danger of touching the ground is reduced. It might be
concluded that the foot is lifted no more than strictly required,
resulting in the observed reduced knee flexion at higher speeds.
2168 M. VERSTAPPEN, P. AERTS AND R. VAN DAMME
However, during running, the vertical oscillations of the centre
of mass remain very small (groucho running; McMahon,
1985), and reduced knee flexion can be expected finally to
result in the feet touching the ground, unless alterations in the
relative timing of changes in the hip and ankle angles
compensate for this negative effect. However, these angles
show no significant velocity-related changes. It may be that the
minimal knee angle during the recovery phase is highly
variable from step to step, being determined by the details of
the take-off conditions. Given the light weight of the leg
segments distal to the knee, the leg configurations can have
only limited effects on the swing characteristics of the leg. If
this is true, the significant relationship between knee angle and
locomotor speed during recovery might be an accidental
consequence of the limited number of trials.
The joint movements described here for magpies are
comparable with those of other bird species such as the chicken
(Jacobson, 1980; Jacobson and Hollyday, 1982a; Johnston and
Bekoff, 1992; Manion, 1984; Muir et al., 1996), the pigeon
(Cracraft, 1971), the guineafowl (Gatesy, 1999a, only hip and
knee) and the silver gull Larus novaehollandiae (Dagg, 1977,
only the ankle). In two studies of the chicken (Jacobson and
Hollyday, 1982a; Johnston and Bekoff, 1992), the ankle angle
is fairly constant during the stance phase, while in the magpie,
in other studies of the chicken (Jacobson, 1980; Manion,
1984), in the pigeon (Cracraft, 1971) and in the silver gull
(Dagg, 1977), there are clear flexion and extension movements.
Gatesy (1999a) found that the pelvic pitch (the position of the
trunk with respect to the horizontal) of walking/running
guineafowl decreased with increasing velocity. In magpies,
pelvic pitch changes very little when the birds change from a
walk to a run or a hop (see Table 3).
Birds are digitigrade, i.e. they stand on their toes. Toe
morphology shows adaptations to the habitat utilised by a
species. For example, in running birds such as the chicken, the
hallux is reduced in size (see Figs 42 and 43 in Manion, 1984),
and in the ostrich, it is no longer present (Alexander, 1985).
Magpies, like most other birds, are facultatively terrestrial, so
their toes have an important role in perching. The four toes are
well developed and are placed in a configuration of three front
toes and one hind toe (hallux) (Verstappen et al., 1998). The
external toe angle is defined as the angle between the sole of
the toes and the horizontal, and the internal toe angle is the
angle between the front toes and the hallux (Figs 1A, 7).
During stance, the internal toe angle is maximal. As the ankle
starts to flex at the onset of recovery, tension will act on the
tendons of the toe flexors such that, as soon as the toes leave
the ground, rapid toe flexion occurs. Halfway through the
recovery, the ankle joint and the toes start to extend. Since
ankle extension unstrains the toe flexor tendons, toe extension
can occur passively. It cannot, however, be excluded that this
passive extension is aided by activation of the musculus
extensor digitorum longus (extension of front toes) and the
musculus extensor hallucis longus (extension of the hallux). In
contrast to the chicken (Jacobson, 1980), the toes are not set
down simultaneously. In the pigeon (Cracraft, 1971) and the
magpie, the hallux touches the ground just before the front toes
do, followed by contact of the plantar surface of the foot with
the ground (see Fig. 7). During the present experiments, the
running track was covered with a thin layer of cork. Variations
in the movement pattern of the toes may occur because of the
nature of the surface on which the birds move.
Humans are also efficient bipeds. Although humans and birds
use the same type of locomotion and show adaptations to it,
differences in leg morphology are striking. It is common
knowledge that elongation of the distal leg segments increases
the step/stride length, which in turn leads to the possibility of
velocity increasing. Birds use this mechanism to a great extent.
(i) In birds, because of the relatively short femur, the knee is
located next to the trunk. (ii) The ankle joint in humans lies
between the tibia/fibula and the tarsal bones. In birds, the tibia
is fused with the proximal tarsal bones, and the metatarsal bones
are fused with the distal tarsal bones. Therefore, the ankle lies
between the elongated tibiotarsus and tarsometatarsus and
bends in the same way as the ankle does in humans. The
position of a bird’s ankle in the leg is comparable with that of
a human’s knee. (iii) Although a human foot is relatively long,
it does not add to the leg length. The foot configuration in birds
is changed such that the leg length is increased markedly by the
foot. The elongated tarsometatarsus does not touch the ground
during the stance phase, only the toes (phalanges) do. By
moving on the tips of their toes, birds add both length and an
important joint (interdigital joint, see for example Verstappen
et al., 1998) to the leg.
The overall patterns of changes in internal and external
angles in walking, running and hopping magpies were
relatively constant between trials and even between gait types,
in contrast to the large inter- and intra-individual variation
found for the hip and ankle angles in walking humans
(Borghese et al., 1996). The major patterns of change in
external angles are comparable between magpies and humans
(Borghese et al., 1996). However, for the internal angles, some
differences are seen. In magpies, the hip angle extends
throughout the stance phase. In humans, this is not always the
case because, in some individuals, the hip shows a
flexion–extension movement (Borghese et al., 1996). At touchdown, a human slightly flexes the knee, then extends it again;
near the start of recovery, another flexion of the knee occurs
(Borghese et al., 1996; Vaughan, 1984; Winter, 1984). In
magpies, the knee flexes throughout the stance phase
(sometimes with a small extension, see above). At touch-down,
the human ankle continues to extend slightly as a result of the
previous recovery phase, but then flexes, extending again
towards the start of the recovery phase (Borghese et al., 1996;
Vaughan, 1984; Winter, 1984). When humans run, the ankle
starts to flex immediately at touch-down (Vaughan, 1984). In
magpies, this maximal ankle angle can occur at the transition
from the recovery to the stance phase or at the beginning of
the stance phase. There was no significant relationship between
the relative timing of maximal ankle angle and velocity. Upon
recovery, the human ankle flexes markedly and then extends
again, but the extension is less pronounced than during the
Kinematics of bipedal gaits in magpies 2169
stance phase (Borghese et al., 1996; Vaughan, 1984; Winter,
1984). In magpies, the extensions towards the end of the stance
and the recovery phases were of equal magnitude. The
differences between humans and magpies are probably caused
by the relative positions of the joints in the leg (see above) and
by posture. The leg movements in birds are mainly influenced
by extension and flexion of both the knee and ankle
(movements of the tibiotarsus and tarsometatarsus); in humans,
leg movements rely mostly on the angular displacements of the
hip and knee (movements of the femur and tibia/fibula). The
posture of the trunk differs between humans and magpies, and
this probably causes differences in angular displacements of
the legs and their segments. Humans keep their trunk upright,
whereas in magpies it is at an inclination of approximately 45 °
with the horizontal. In both bipeds, the trunk is held relatively
stable during locomotion.
Movements of the leg segments result, of course, in
movements of the entire leg. Legs can be protracted (move
forwards) or retracted (move backwards). In contrast to
humans, who increase the protraction angle of their legs with
velocity within a gait type (Gatesy and Biewener, 1991),
magpies keep this angle fairly constant with increasing velocity
and even over the different gaits. Gatesy (1999a) reports a
constant protraction angle of approximately 46 ° for walking
and running guineafowl, which agrees well with the values
found here for walking, running and hopping magpies
(approximately 45 °). The only exception is for the take-off leg
during hopping. The protraction angle of this leg is somewhat
larger (approximately 53 °). The magnitude of the protraction
angle could be limited by the small angular excursions of the
hip in magpies.
Running and out-of-phase hopping
In addition to the differences in leg morphology, there are
noticeable differences in the use of the three gait types between
magpies and humans. At a Froude number (dimensionless
velocity, Alexander, 1992) of approximately 0.5, magpies and
humans stop walking and switch to a faster gait (Verstappen
and Aerts, 2000; Alexander, 1992). Whitall and Caldwell
(1992) described the spatio-temporal gait characteristics and
kinematics of running and galloping in humans. They found
that, when humans increase their speed, they change from
walking to running. In some circumstances, galloping occurs
(also known as skipping, see Minetti, 1998), but running is
definitely the preferred gait. The voluntary speed achieved by
galloping humans is generally lower than that during running
(Caldwell and Whitall, 1995). The authors argued that the
relationship between running and galloping in humans is not
directly comparable with analogous relationships existing in
other animals. Verstappen and Aerts (2000) showed that
magpies walk at low speeds and that, like humans, they change
their gait to either running or hopping at higher speeds. Two
differences between magpies and humans concerning the gait
types selected are noteworthy: (i) magpies prefer hopping at
higher speeds and (ii) the voluntary speed of running in
magpies is lower than that of hopping.
Why magpies prefer hopping and why running is used at
lower speeds than hopping cannot be determined from the
present kinematic results because we have found that running
and hopping are very similar gait types. Future research
recording ground reaction forces might provide answers to
these questions.
M.V. is funded by IWT-grant SB961234. P.A. is Research
Director of the Fund for Scientific Research-Flanders. R.V.D.
is post-doctoral researcher for the Fund for Scientific
Research-Flanders.
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